The industries relating to optical elements and fused quartz crucibles have a continuing need for high purity silica particles as a source of fused quartz. For example, fused quartz or silica crucibles are used for growing silicon crystals which find use in electronic circuits. Necessarily, these crucibles must be of a high purity to ensure pure crystal growth. Silica or quartz that is currently used in the manufacture of fused silica crucibles is a natural quartz of very high purity and is available in only very limited number of places in the world. Its particle size is critical and is controlled between 50 to 150 mesh. From a purity viewpoint, natural quartz has some inherent limitations and ever increasing purity requirements of the electronics and optical industries demand that crucibles be made from purer and purer starting material. Synthetic silica can fill this purity gap. However, commercially available synthetic silica powder is very light, fluffy, fine and generally not suitable for making crucible or other fused quartz products.
Optical elements also must be free of bubbles, color centers and elements which can absorb radiation in undesired regions of the wavelength spectrum.
The requirements for each of the above applications demand that the silica particles to be fused have a relatively high density. If the density is too low, air trapped within the pore structure forms bubbles upon heating the silica particles. These bubbles do not escape due to the high viscosity of the molten glass. Since high density silica feeds reduce bubble formation, there is a need to make dense, high purity, synthetic silica particles of the desired size which can be used for crucible manufacture as well as for other applications in the optical industry. To the knowledge of inventors, there is no known commercial method of making coarse and dense silica powders of purity suitable for above applications.
Many impurities can be introduced during formation of a silica particle. One production method uses a mixture of sodium silicate and either an acid or a base to form a silica gel. This gel is then washed and spray dried to make a dried, relatively low density product. In one variation of the production method, silica gel material may be formed directly into fused silica without an intermediate washing or spray drying. For example, see U.S. Pat. Nos. 4,063,916 and 3,838,998. In another processing technique silicon tetrachloride is hydrolized in a flame and deposited directly on a mandrel or "bait". As an example of such a process, see U.S. Pat. No. 3,806,570. A similar technique can also be applied to a plurality of different types of metal chloride gases. See, U.S. Pat. No. 3,801,294.
Silica particles, known as silica flour, generally have has a particle size of up to about 70 microns. Silica flour particles are amorphous and have a bulk density of only about 0.2 to 0.4 g/cc.
On the other hand, completely fused silica particles provide a glass product having a bulk density of about 2.2 g/cc. One method for producing such completely fused glass from a silica flour uses an electric arc through which the silica flour is dropped. See, U.S. Pat. Nos. 3,151,964 and 3,553,756. This glass product is very hard and is almost impossible to break by thermal shock. These properties make the glass difficult to grind into powder because the particles abrade the grinding elements thereby introducing impurities into the resultant product.
It would be desirable to produce an aggregate of bonded fused glass particles that can be ground in a relatively simple manner without introducing impurities into the ground product.